Language packs: Remove translations when deleting a theme or a plugin.

This is for translation files in WP_LANG_DIR which are installed through a language pack.
Change `wp_get_installed_translations()` to only return a translation if the .mo file also exists.

fixes #29860.
Built from https://develop.svn.wordpress.org/trunk@29856


git-svn-id: http://core.svn.wordpress.org/trunk@29619 1a063a9b-81f0-0310-95a4-ce76da25c4cd